A polypropylene resin in-mold expanded molded product, which is obtained using expanded polypropylene resin particles containing a polypropylene resin, has characteristics of, for example, being formed in any shape, being light in weight, and being heat insulating, which characteristics are advantages of an in-mold expanded molded product.
In comparison with a polystyrene resin in-mold expanded molded product which is obtained by use of expanded polystyrene resin particles, a polypropylene resin in-mold expanded molded product is superior in, for example, chemical resistance, heat resistance, and strain recovery rate after compression. In comparison with a polyethylene resin in-mold expanded molded product which is obtained using expanded polyethylene resin particles, a polypropylene resin in-mold expanded molded product is superior in, for example, dimension accuracy, heat resistance, and compressive strength.
A polypropylene resin in-mold expanded molded product, which has the above characteristics, is variously used for, for example, not only automobile interior materials and automobile bumper core materials but also heat insulating materials, shock-absorbing packing materials, and returnable containers.
As described above, a polypropylene resin in-mold expanded molded product is superior to a polyethylene resin in-mold expanded molded product in heat resistance and compressive strength. However, according to a polypropylene resin in-mold expanded molded product, a molding temperature during in-mold foaming molding becomes higher, and a high steam pressure is necessary during, for example, in-mold foaming molding by use of steam. This tends to cause an increase in utility cost.
There have been proposed the following techniques: (i) a technique in which a low-melting polypropylene resin having a melting point of less than 145° C., and further not more than 140° C. is used (e.g., Patent Literature 1 or 2); (ii) a technique in which a mixture of a polypropylene resin having a low melting point and a polypropylene resin having a high melting point is used (e.g., Patent Literatures 3 through 8); (iii) a technique in which a metallocene polypropylene resin having a low melting point and polymerized by use of a metallocene catalyst is used (e.g., Patent Literature 9); and (iv) the like.
However, even in a case where a molding temperature can be reduced by these techniques, in-mold expanded molded products produced by these techniques exhibit a lower compressive strength and a lower surface property of a molded product edge part than conventional in-mold expanded molded products.
Specifically, for example, in a case where a polypropylene resin in-mold expanded molded product for an automobile bumper has a molded product density of 30 g/L, the polypropylene resin in-mold expanded molded product is required to have a strength of approximately 0.23 MPa as a compressive strength when the polypropylene resin in-mold expanded molded product is strained by 50% (hereinafter referred to as “50%-strained compressive strength”). According to a conventional technique, a pressure of not less than 0.26 MPa (gauge pressure) as an in-mold foaming molding pressure (and a high molding temperature) is (are) necessary for obtainment of a polypropylene resin in-mold expanded molded product having the above strength.
Meanwhile, in a case where (i) a polypropylene resin having a low melting point, (ii) a mixture of a polypropylene resin having a low melting point and a polypropylene resin having a high melting point, or (iii) a metallocene polypropylene resin polymerized by use of a metallocene catalyst is used, an in-mold expanded molded product can be molded at an in-mold foaming molding pressure of not more than 0.20 MPa (gauge pressure). However, the in-mold expanded molded product has a 50%-strained compressive strength that is far below 0.23 MPa.
A decrease in compressive strength in the case of use of a polypropylene resin that can be molded at such a low molding pressure (and molding temperature) tends to be significant in a case where a molded product has a molded product density of not more than 40 g/L.
A balance among a low in-mold foaming molding pressure, a high compressive strength, and a good surface property is to be achieved by appropriately regulating a melting point of a polypropylene resin so as to increase a 50%-strained compressive strength while keeping an in-mold foaming molding pressure low. However, a molded product edge part has a lower surface property, so that the balance is difficult to achieve well.
A metallocene polypropylene resin has a higher production cost than a Ziegler polypropylene resin polymerized using a Ziegler catalyst. Therefore, even in a case where utility costs of in-mold foaming molding can be reduced as a result of a decrease in molding temperature, material costs are still high. In view of this, the metallocene polypropylene resin is not necessarily industrially advantageous.
Besides the above techniques, there is known a technique for providing expanded polypropylene resin particles which (i) allow an in-mold expanded molded product to be produced at a low heating steam pressure, (ii) cause the in-mold expanded molded product thus produced to have a low shrinkage percentage, and (iii) are highly beautiful in surface appearance (Patent Literature 10). According to the technique, an in-mold expanded molded product that has a low in-mold foaming molding pressure, a low shrinkage percentage, and a good surface property can be obtained by specifying an MFR, a melting point, and a flexural modulus of a polypropylene resin. However, the technique has a restriction under which (i) a polypropylene resin that satisfies an expression defined by “[Flexural modulus (MPa)]≥31.19×[Melting point (° C.)]−3500” needs to be used as a base material resin and (ii) the melting point needs to be not more than 145° C. Thus, the technique has room for improvement from the viewpoint of efficiently using a base material resin that is not bound by such a restriction.
Under the circumstances, there are still demands for a technique for achieving a high-compressive-strength polypropylene resin in-mold expanded molded product beautiful in surface appearance while reducing a molding temperature during in-mold foaming molding.